The present invention relates to a seat belt retractor installed in a vehicle such as an automobile for winding a seat belt on a spool with driving torque of a motor to restrain and protect an occupant, and more particularly, to a seat belt retractor in which a load torque above a predetermined value is not transmitted from a spool to a driving torque transmission mechanism that transmits the driving torque of the motor to the spool. The present invention also relates to a seat belt device provided with the seat belt retractor.
Conventionally, a seat belt device is installed in a vehicle such as an automobile, and a seat belt restrains an occupant so that the occupant does not move out of a vehicle seat, thereby protecting the occupant in case of emergency such as a vehicle collision in which large deceleration is exerted on the vehicle. The seat belt device is provided with a seat belt retractor for urging a spool with the seat belt wound thereon in a direction of winding the belt all the time. The seat belt retractor locks the spool not to rotate in a direction of unwinding the seat belt when an emergency locking mechanism (ELR mechanism) is activated in case of emergency as mentioned above, thereby preventing the seat belt from being withdrawn.
As a conventional seat belt retractor, Japanese Utility Model Publication (Kokai) No. 61-134465 has disclosed a seat belt retractor in which a motor drives a spool with driving torque to rotate to wind a seat belt. In such a seat belt retractor, it is possible to control a tension of the seat belt with the motor for winding the seat belt. Accordingly, it is possible to select various modes of winding the seat belt, thereby adjusting the seat belt to restrain an occupant according to a condition such as a vehicle running condition.
In the conventional seat belt retractor as disclosed in Japanese Utility Model Publication (Kokai) No. 61-134465, when the automobile collides lightly in an extent that an ELR mechanism is not activated while the motor is winding the seat belt, the seat belt is withdrawn due to inertia of the occupant so that tension may be applied to the seat belt.
Accordingly, a load due to energy of the tension on the seat belt is transmitted to the spool. When the load is transmitted to a driving torque transmission mechanism via the spool when the motor rotates the spool in a direction of winding the belt, a load torque larger than that in a normal state acts on a component such as a gear composing the driving torque transmission mechanism. Accordingly, it is necessary to increase strength of the component such as a gear in the driving torque transmission mechanism, so that the component withstands such large torque. Further, the motor needs to generate the driving torque against the load transmitted to the driving torque transmission mechanism, thereby increasing load on the motor.
In view of the problems described above, the present invention has been made, and an object of the present invention is to provide a seat belt retractor driven with a motor in which load torque larger than a predetermined level is not transmitted to a driving torque transmission mechanism when the motor drives. Another object of the present invention is to provide a seat belt device provided with the seat belt retractor.
Further objects and advantages of the invention will be apparent from the following description of the invention.